In gas-turbine engines, the compressor experiences two main types of instabilities, known as rotating stall and surge. Rotating stall is a flow separation that travels around the annulus of the compressor (referred to as a stall cell). Typical effects associated with stall include high mechanical stress level in the blading, large drop in performance, and possible turbine overheating due to the decreased flow. Furthermore, the rotating stall condition is irrecoverable in some systems due to the presence of a hysteresis loop, in which case a shut-down and restart of the entire engine is in order. Surge is a large flow oscillation in the compression system which induces high blade and casing stress levels and possible reverse flow which is detrimental to combustion and engine performance.
Active control of rotating stall and surge can lead to an increase in the stability of a compressor against various disturbances such as inlet distortions and power transients. As a result, a compressor with active controls can operate closer to the current stall/surge line.
Active control of rotating stall and surge has been modeled and tested by various researchers. Successful attempts at stabilization have been achieved using inlet guide vanes (IGV), bleed valves (BV), and air injection (AI) on a variety of research compressors. A simplified model was derived by Moore and Greitzer for a compressor that exhibits rotating stall and surge. Based on this model, Liaw and Abed derived a control law using a bleed valve for rotating stall. Attempts at control of rotating stall on a single-stage, low speed axial compressor at Caltech were carried out initially with a high speed bleed actuator and results were unfruitful due to the fast growth rate of the stall cell relative to the rate limit of the valve. For industrial applications where the compressors may be significantly more powerful (higher flow and pressure rise, higher rotor frequency, etc.) than research compressors, obstacles such as control actuator magnitude and rate saturation can become crucial in these active control methods. A method which reduces the rate requirements of actuators for purposes of active control of rotating stall in compressors can be valuable in circumventing possible actuator rate limitations that prevents successful active control implementation.